A communication method of transmitting a signal to a terminal using air as a medium has been known. This method uses an existing speaker corresponding to a pronunciation component and a microphone installed in a terminal such as a mobile phone (Japanese Patent No. 4295781 and the like).
Recently, transmission of data of program information from a speaker of a television, and transmission of data by a distance of about several meters from a small health care device have been under consideration as applications of the communication method.
Incidentally, a general speaker considered to be used in the communication method is mainly a dynamic type, and there is considerable difficulty in miniaturization, weight lightening, and power saving in this dynamic speaker.
For this reason, a piezoelectric module is preferable in miniaturizing a pronunciation component. The piezoelectric module easily achieves miniaturization, weight lightening, and power saving when compared to a dynamic speaker as disclosed in Japanese Unexamined Patent Publication No. H11-52958. For example, case dimensions of 13 mm or less in diameter and 8 mm or less in height may be achieved as a small piezoelectric module.
In such a piezoelectric module, a frequency band of 16 to 20 kHz is suitable for short distance transmission. At a lower frequency than 16 kHz, sound is easily heard, and thus this frequency is unsuitable for communication. In addition, at a higher frequency than 20 kHz, a data transfer rate is low, and thus this frequency is unsuitable for data communication.
However, when the above-described small piezoelectric module is employed as a pronunciation component, an unstable communication state, in which some signals do not arrive at a microphone for reception positioned at a predetermined distance (show distance), is generated in the frequency band of 16 to 20 kHz in some cases.
The inventors have conducted a keen examination on stability of this communication, and have found that a cause thereof is related to a resonance point of a piezoelectric module. That is, the inventors have found that, when a resonance point of a piezoelectric module is within a frequency band of a transmitted signal, a signal transmission distance is noticeably shortened at the resonance point. In this regard, the inventors have proposed a technology that attempts improvement in stability of communication by excluding a resonance point from a frequency band of a transmitted signal to avoid shortening of a signal transmission distance (see Japanese Patent No. 5578218).
However, a resonance space of a piezoelectric module hinders further miniaturization of the piezoelectric module, and thus a piezoelectric module not including any resonance space is desired.